Computer Modeling Yields Clues to How the Brain Works

A computer simulation developed by
Salk researchers led by investigator Terrence J.
Sejnowski reveals that neurons in the thalamus,
the central switchboard that processes and
distributes incoming sensory information to all
parts of the visual cortex, engage in a coordinated
effort to get their message out loud and clear.

Their findings, published in Science, hold
important clues to how the brain encodes and
processes information, which can be applied to
a wide variety of applications, from understanding
psychiatric disorders to the development
of novel pharmaceuticals and new ways of
handling information by computers or
communication networks.

Historically, neuroscientists have been limited
to recording the activity of single brain cells,
but communication between neurons is not
limited to one-on-one interactions. Instead, any
Computer Modeling Yields Clues to How the Brain Works
given cell receives signals from hundreds of
other cells, which send their messages through
thousands of synapses.

For this reason, nobody could answer a very
basic question: How many neurons or synapses
does it take to reliably send a signal from point A
to point B? This question is particularly pressing
for the thalamus. Thalamic input only accounts
for five percent of the signals that
so-called spiny stellate cells in the cortex
receive, even though they drive a good portion
of activity throughout the cerebral cortex.

"That is a paradox," says Sejnowski, a
Howard Hughes Medical Institute investigator
and professor and head of the Computational
Neurobiology Laboratory. "How can so few
synapses have such a big impact? If the
average spiking rate were the determining
factor, thalamic input would be drowned out
by the other 95 percent of the inputs from
other cortical cells."

Based on the assumption that the brain
cares about the reliability and precision of
spikes, Sejnowki's team developed a realistic
computer model of a spiny stellate cell and the
signals it receives through its roughly 6,000
synapses. They found that it is not the number
of spikes that's relevant but rather how many
spikes arrive at the same time.

The team's model predicted that it only
takes about 30 synapses out of 6,000 firing
simultaneously to create extremely reliable
signaling. And the prediction lined up with
currently available in vivo measurements.
The researchers hope that their findings will
give them new insight into the holy grail of
neurobiology: decoding the neural code or
language of the brain.